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    mayflower77's Avatar
    mayflower77 Posts: 1, Reputation: 1
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    #1

    May 5, 2009, 06:53 PM
    How to find focus, when the vertex is know
    (x+4)^2=16(y+2) find the vertex, focus, and directix of the parabola. Graph the equation. I got the vertex, how do find the focus and directix.
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
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    #2

    May 6, 2009, 05:23 AM

    Parabola - Wikipedia, the free encyclopedia gives you a lot of information about parabolas.

    Basically, if you arrange the equation for a parabola in this form:



    or



    The vertex is (x,k); the focus is (h, k+p); the directrix is y=k-p where p is the distance from the vertex to the focus.

    Is your equation in this form?



    If it is, you should be able to pick out the values directly. The + and/or - signs shouldn't throw you. If you had (x+7)^2, h is simply -7.
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